Uncheck option "Automatic Page Settings" on tab "PDF" in the PDF Export settings. With "Automatic Page Settings" deactivated, you can manually specify the page sizes of the exported PDF document. Note, though, if the size and scaling settings on tab "Clipping" result in diagram dimensions that exceed the manually chosen PDF page size, your diagram will be split over several pages.
That said, even with manually specified page sizes, it is not possible to guarantee your requirement. E.g. suppose you decide on A4 as PDF page size. Suppose further the user printing the document only has US Letter size paper. Now, PDF printing software typically supports automatic scaling of documents to fit the printer's supported paper size. If the aforementioned user has this option enabled, the size of your document (and thus the size of the displayed shapes) is changed during the actual printing process. There is nothing that can be done about that when creating the document.
You could try to instruct your users to turn off their PDF printing software's automatic scaling and hope for the software to simply cut of the document's margins. But the success of this approach would depend on the technical knowledge of your users, their PDF printing software, and probably their actual printer and printer driver. (In short, a lot of factors you probably cannot control.)
